This invention relates to magnetic tunnel junction (MTJ) devices for memory and external magnetic field-sensing applications. More particularly the invention relates to a MTJ device that uses an improved insulating tunnel barrier material that improves the properties of the MTJ.
A magnetic tunnel junction (MTJ) is comprised of two layers of ferromagnetic material separated by a thin insulating tunnel barrier layer. The insulating layer is sufficiently thin that quantum-mechanical tunneling of the charge carriers occurs between the ferromagnetic electrodes. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments (magnetization directions) of the two ferromagnetic layers. The two ferromagnetic layers are designed to have different responses to magnetic fields so that the relative orientation of their moments can be varied with an external magnetic field. The MTJ is usable as a memory cell in a nonvolatile magnetic random access memory (MRAM) array, as described in IBM""s U.S. Pat. No. 5,640,343, and as a magnetic field sensor, such as a magnetoresistive read head in a magnetic recording disk drive, as described in IBM""s U.S. Pat. No. 5,729,410.
FIG. 1 illustrates a cross-section of a conventional MTJ device. The MTJ 10 includes a bottom xe2x80x9cfixedxe2x80x9d ferromagnetic (FM) layer 18, an insulating tunnel barrier layer 20, and a top xe2x80x9cfreexe2x80x9d FM layer 32. The MTJ 10 has bottom and top electrical leads, 12, 14, respectively, with the bottom lead 12 being formed on a suitable substrate. The FM layer 18 is called the xe2x80x9cfixedxe2x80x9d layer because it is formed of a high-coercivity material whose magnetic moment (magnetization direction) is prevented from rotation in the presence of applied magnetic fields in the desired range of interest for the MTJ device, i.e., the magnetic field caused by the write current applied to the memory cell from the read/write circuitry of the MRAM or the magnetic field from the recorded magnetic layer in a magnetic recording disk. The magnetic moment of FM layer 18 can also be fixed by being exchange coupled to an antiferromagnetic layer. The magnetic moment of the free FM layer 32 is not fixed, and is thus free to rotate in the presence of an applied magnetic field in the range of interest. In the absence of an applied magnetic field the moments of the FM layers 18 and 32 are aligned generally parallel (or antiparallel) in a MTJ memory cell and generally perpendicular in a MTJ magnetoresistive read head. The relative orientation of the magnetic moments of the FM layers 18, 32 affects the tunneling current and thus the electrical resistance of the MTJ device.
What is important for MTJ device applications is the signal-to-noise ratio (SNR). The magnitude of the signal is dependent upon the magnetoresistance or MR (xcex94R/R) exhibited by the device. The signal is given by iB xcex94R, which is the bias current (iB) passing through the MTJ device (assuming a constant current is used to detect the signal) times the resistance change (xcex94R) of the device. However, the noise exhibited by the MTJ device is determined, in large part, by the resistance R of the device. Thus to obtain the maximum SNR for constant power used to sense the device the resistance (R) of the device must be small and the change in resistance (xcex94R) of the device large.
The resistance of a MTJ device is largely determined by the resistance of the insulating tunnel barrier layer for a device of given dimensions since the resistance of the electrical leads and the ferromagnetic layers contribute little to the resistance. Moreover, because the sense current passes perpendicularly through the ferromagnetic layers and the tunnel barrier layer, the resistance R of a MTJ device increases inversely with the area A of the device. The requirement for low resistance MTJ devices, coupled with the inverse relationship of resistance with area, is especially troublesome because an additional requirement for MTJ device applications is small area. For an MRAM the density of MTJ memory cells in the array depends on small area MTJs, and for a read head high storage density requires small data trackwidth on the disk, which requires a small area MTJ read head. Since the resistance R of a MTJ device scales inversely with the area A, it is convenient to characterize the resistance of the MTJ device by the product of the resistance R times the area A (RA). Thus RA is independent of the area A of the MTJ device.
In prior art MTJs, the material used for the tunnel barrier layer is aluminum oxide (Al2O3) because such barrier layers can be made very thin and essentially free of pin holes. For Al2O3 barrier layers it has been found that RA increases exponentially with the thickness of the layer. The thickness of Al2O3 barrier layers can be varied over a sufficient range to vary RA by more than eight orders of magnitude, i.e., from more than 2xc3x97109 xcexa9(xcexcm)2 to as little as 20 xcexa9(xcexcm)2. However, for these lower resistance values, the MR is typically reduced, most likely because of the formation of quantum point defects or microscopic pin holes in the ultra thin tunnel barrier layers needed to obtain these very low RA values. For MRAM applications RA values in the range 500-1000 xcexa9(xcexcm)2 are acceptable, although it would be useful to be able to prepare MTJ memory cells with even lower RA values so that, for example, current could be passed perpendicularly through the MTJ cell to aid in the writing of the cell. Moreover, for scaling to ever higher memory capacities, MRAM cells will need to be shrunk in size, requiring lower RA values so that the resistance of the cell is not too high. More importantly, for MTJ read heads to be competitive in SNR with conventional giant magnetoresistance (GMR) spin-valve read heads, the MTJ heads need to have resistance values comparable to those of GMR heads. Since read heads of sub-micron size are required for high density recording applications, MTJ heads with RA values lower than those which can be obtained with Al2O3 tunnel barriers are needed. For otherwise the same size MTJ device, read heads typically require lower RA values that MRAM cells. This is because in the operation of the MRAM cell only two states of the cell need to be distinguished, where the magnetic moments of the ferromagnetic layers in the device are either parallel or antiparallel to one another. By contrast, for read heads, the response of the device must be monitored continuously over a wide range of states of the device.
Thus, it is desirable to develop MTJ devices with lower RA values than can be achieved with MTJ devices that use conventional Al2O3 tunnel barriers, and where the lower RA values can be achieved without sacrificing high MR.
The invention is a magnetic tunnel junction device with a tunnel barrier made of a material consisting essentially of an oxide or nitride of one or more of gallium and indium. An oxide or nitride of aluminum may be included as part of this tunnel barrier material. In one embodiment the tunnel barrier is an oxide of a gallium-aluminum alloy (Ga75Al25). The Ga oxide tunnel barrier may be formed by sputter deposition of Ga, followed by a plasma oxidation, or by depositing Ga from an effusion source in the presence of oxygen gas or in the presence of more reactive oxygen provided by an atomic oxygen source or other source. The tunnel barrier layer may also be formed as a bi-layer structure with an aluminum oxide layer formed directly on one of the ferromagnetic layers of the device, followed by a gallium oxide layer formed directly on the aluminum oxide layer. By appropriate selection of the amounts of gallium and or aluminum, or the thicknesses of the aluminum oxide and gallium oxide in the bi-layer structure, the tunnel barrier energy height can be tuned to a selected value. The magnetic tunnel junction devices made with the improved tunnel barrier material show a substantially reduced tunnel barrier energy height (and thus lower RA values) compared to conventional devices using aluminum oxide tunnel barriers, without a reduction in magnetoresistance.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the following detailed description taken together with the accompanying figures.